In extracting oil and gas from a formation, it is advantageous to monitor the flow rates of the different components of the production fluid, usually gas, oil and water. Such a three-component mixture presents a problem that is often more difficult to solve than that of determining the flow rates for binary mixtures. For a broad class of well mixed binary mixtures flowing in a conduit of known cross sectional area, for which the density and the sound speed of each component are known, measuring the sound speed of the mixture and the convection velocity of the mixture provides sufficient information to determine the flow rates of each of the components. The sound speed of the mixture is directly related to the phase fractions of each component in the mixture. More specifically, to determine the four unknowns, the phase fraction of each component and the flow rate of each component, four equations are used. One equation expresses that the sum of the component (phase) fractions is unity. Another is that the speed of sound in the mixture depends on the speed of sound and density of the individual components and the phase fraction of each constituent. Finally, there are two equations relating the flow velocities of the individual components to the overall mixture flow rate.
The approach of using a single sound speed measurement and a mixture flow rate measurement used in determining the component velocities in the case of a binary mixture, however, cannot be extended to solve the problem of finding the component velocities of a fluid with more than two components. Additional information is required beyond what is provided by a measurement of the speed of sound in the mixture and a measurement of the flow rate of the mixture.
To provide the required additional information, the prior art teaches multiphase flow meters that typically rely on several so called orthogonal measurement systems (said to be a orthogonal because each measurement system provides information that is at least partially independent of the information provided by the other measurement systems). Multiphase flow meters according to such an orthogonal system approach include meters based on multiple-energy nuclear sources, ultraviolet measurements, capacitance measurements, venturi effect measurements, and infrared measurements.
An alternative approach to determining the component flow velocities of a multiphase fluid in a conduit is to determine the additional required information from multiple point measurements, i.e. from measurements of the same information at different places along the conduit. For example, the speed of sound in the fluid, the flow rate of the fluid, and the pressure and temperature of the fluid would be made at two or more locations along the conduit. In addition, a multiphase flow model is used to relate the values of the parameters that are measured at one location to those measured at another location. These relationships can provide the additional constraints required to solve for additional parameters. Typically, the equations are nonlinear in several variables. Many methods can be employed to “solve” for the flow parameters of interest.
One class of methods defines an error function based on the -calculated values of the parameters compared to the measured values of the parameters. The flow-related parameters sought (i.e. the component velocities or component phase fractions) are adjusted -iteratively until the error is minimized. In this context, the flow parameters that result in minimizing the value of error function are assumed to be correct.
One company, previously Loke of Norway (now owned by FMC/KOS), is considered by many to have pioneered the general approach based on multiple point measurements for production allocation measurements in oil and gas production facilities. One implementation by Loke of the general approach is their software called Idun, which uses conventional pressure and temperature measurements along with a choke position measurement and knowledge of the fluid property characteristics to estimate the component flow rates (or phase fractions). The overall accuracy and robustness of the Idun approach is directly influenced by the type and quality of sensors available.
Another approach to determining component flow rates in a multiphase fluid based on making measurements at multiple locations along a conduit carrying the multiphase fluid is that based on a gradio-venturi system, which includes a venturi meter and employs a remote pressure sensor located several hundred feet above the venturi. The pressure difference between the pressure at the venturi and that at the remote transducer can be related to the flow rate and composition through a multiphase flow model, and can be used in conjunction with the pressure difference due to the flow thorough the venturi to estimate the component flow rates. Such an approach has several drawbacks. It requires a venturi, which is intrusive, and it has an accuracy limited in two-phase flow to ±10% of the total flow. Moreover, the accuracy degrades substantially in the presence of any significant entrained gas.
What is needed is a system of measuring the component velocities and phase fractions of a fluid that includes at least three components, and that is nonintrusive, sufficiently accurate, and that does not provide spurious solutions because of insufficient information.